Elastic connecting rods are typically used in conjunction with pedicle screws for the dynamic stabilization of the spine. If such a rod can be provided with a section of higher stiffness, the possible indications can be substantially extended. Thus, a section of higher stiffness may be used for the fusion of vertebral bodies while a section of lower stiffness may be used for the elastic connection of a neighboring vertebral body.
Today, the “golden standard” in spinal surgery still consists in the fusion (stiffening) of pathologic vertebral bodies. However, fusions often lead to premature degeneration of the segments adjacent to the fusion. Therefore, attempts are being made to provide rods having different degrees of stiffness and to include neighboring segments in the surgical treatment.
The approaches of the prior art to solving this problem will be set forth below.
The invention according to patent application US 2008/0319486 discloses a connecting element having a variable stiffness along its longitudinal axis. The variable stiffness is limited to the sections between two anchoring elements and provides a different response to pressure only.
The invention according to EP 2 113 216 also provides different degrees of stiffness. The connection of the different materials is either achieved by interpenetration or by a butt joint. The material differences are limited to differences in hardness and in bending flexibility. Differences in longitudinal or transversal stiffness are not disclosed. The butt joints are designed for normal stresses exclusively.
The invention according to patent application US 2008/0177388 also connects different rod materials, but this is achieved according to the male/female principle, which is demanding with regard to the production technique.
The invention according to patent application US 2009/0248083 also provides sections of different stiffness, however with a varying core that is difficult to anchor particularly in the stiffer areas.
The invention according to patent application WO 97/32533 provides a varying stiffness due to a varying external diameter, thereby making the rod more difficult to anchor.
The invention according to patent application US 2008/0306536 aims to achieve a variable stiffness between vertebral bodies by connecting the connecting element to the bone screw with variable stiffness. This solution is space-consuming and may have unfavorable consequences for the patient.
The invention according to patent application WO 2007/038429 also allows achieving different degrees of stiffness due to a modular construction but, according to the claims, only in response to bending stresses and not to tensile and compression loads.
The invention according to patent application US 2005/0203513 also provides different degrees of stiffness in that for one part, the core inside a cylindrical wall varies or, for the other part, the stiffness of a connecting element is locally reduced by material removal. On one hand, this entails the difficulty of anchoring a core by means of a wall of a different kind, and on the other hand, the difficulty of sections of predetermined length, which in the case of multisegment treatments involves significant logistic complexity.
The invention according to patent application US 2007/0129729 also provides a stiff and an elastic rod portion whose connection is interpenetrating and is supported by a cable.
The invention according to patent application US 2006/0041259 provides a variable stiffness along its rod axis. The variation is achieved by a helical slot (or spiral slot) reflecting the mechanical principle of a spiral spring. The spring may also be filled with a core and such core may be secured to the spring by welding or bonding. The variation may further be achieved by assembling rods of different properties one behind the other. However, the patent application does not disclose any method of sequentially connecting one rod to the other. Nor does it disclose how the anchorage of the rod in a bone screw would interface with such a connection site.
The inventions according to the patent applications CH 702636 or US 2006/0095134 disclose only blunt connections (butt joint connections) or blunt transitions.
Thus it follows from the prior art that most connections are butt joints, at least peripherally, and thus extend in a plane parallel to the clamping plane in the screw head.